1. Field of the lnvention
This invention relates to a method and apparatus for making transmission holograms and, more specifically, using a rotatable drum adapted to receive a beam of actinic radiation on a reflector in the drum reflecting a portion of the beam through a master to a recording medium to form the transmission hologram in the recording medium.
2. Description of Related Art.
Holography is a form of optical information storage. The general principles are described in a number of references, e.g., "Photography by Laser" by E. N. Leith and J. Upatnieks in Scientific American, 212, No. 6, pages 24-35 (June, 1965). A useful discussion of holography is presented in "Holography", by C. C. Guest, in Encyclopedia of Physical Science and Technology, Vol. 6, pages 507-519, R. A. Meyers, Ed., Academic Press, Orlando, Florida, 1987.
In brief, the object to be photographed or imaged is illuminated with substantially monochromatic light (e.g., from a laser) and a light sensitive recording medium (e.g., a photographic plate) is positioned so as to receive light reflected from the object. This beam of reflected light is known as the object beam. At the same time, a portion of the coherent light is directed to the recording medium, bypassing the object. This beam is known as the reference beam. The interference pattern that results from the interation of the reference beam and the object beam impinging on the recording medium is recorded in the recording medium. When the processed recording medium is subsequently appropriately illuminated and observed at the appropriate angle, the light from the illuminating source is diffracted by the hologram to reconstruct the wavefront that originally reached the recording medium from the object. Thus, the hologram resembles a window through which the real or virtual image of the object is observed in full three-dimensional form.
Holograms formed by allowing the reference and object beams to enter the recording medium from opposite sides are known as reflection holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are, approximately, planes parallel to the plane of the recording medium. When the hologram is played back these fringes act as partial mirrors reflecting incident light back to the viewer. Hence, the hologram is viewed in reflection rather than in transmission.
Holograms that are formed by allowing the reference and object beams to enter the recording medium from the same side are known as transmission holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are approximately normal to the plane of the recording medium. When the hologram is played back by viewing with transmitted light, these fringes refract the light to produce the viewed virtual image. Such transmission holograms may be produced by methods which are well known in the art, such as disclosed in Leith and Upatnieks, U.S. Pat. Nos. 3,506,327; 3,838,903 and U.S. Pat. No. 3,894,787.
The present invention is directed to a method and apparatus for making a transmission hologram.